1. Field of the Invention
The present invention relates to industrial vehicles and to a method for controlling power of the same.
2. Description of Related Art
Conventional forklift trucks use a single engine as a driving source for a loading-and-unloading system and a running system. Accordingly, because an engine having high-output is used, there is a problem of poor fuel efficiency and an increased amount of exhaust gas when the rotation speed of the engine is low.
When a low-output engine is used to address this problem, the method of use thereof is limited in that, for example, it is necessary to separate the running system or the loading-and-unloading system from the engine when the load is high.
In order to overcome these problems, a so-called hybrid system combining an engine and a motor generator has been proposed (for example, see Japanese Unexamined Patent Applications, Publication Nos. 2003-250203 and 2000-313600).
Hybrid systems include series systems that supply power to a running system and a loading-and-unloading system by generating electrical power from a generator rotationally driven by an engine and supplying the obtained electrical power to an electric motor; and parallel systems that supply at least a part of the engine power, as well as power from the electric motor, to the running system and the loading-and-unloading system.
Japanese Unexamined Patent Application, Publication No. 2003-250203 discloses hybrid industrial vehicles for loading and unloading configured to supply power to a running system using a series system, and to a loading-and-unloading system using a parallel system. Japanese Unexamined Patent Application, Publication No. 2000-313600 discloses running vehicles having a loading-and-unloading function configured to supply power to a running system and a loading-and-unloading system using a series system.
In the above method of supplying power using the parallel system, the total power generated from the engine and the electric motor is equal to the total power supplied to the running system and the loading-and-unloading system (power balance is satisfied). The ratio of power generated by the engine to power generated by the electric motor is usually determined by a torque distribution to the engine and the electric motor.
However, a rotation speed of the engine and a rotation speed of the electric motor arbitrarily change with the torque distribution, and rotation speeds of the engine and the electric motor, as well as the torque, are also variables. Therefore, it is complicated to perform control involving many variables for setting the torque so as to satisfy the engine power balance.
The present invention has been conceived to solve the above-described problems, and an object thereof is to provide an industrial vehicle having a hybrid system capable of readily controlling a distribution of power generated by an engine and power generated by an electric motor, and a method of controlling the power of the same.